A liquid crystal display (LCD) device is a display device that controls the transmission/shielding (On/Off of display) of light by controlling the alignment of liquid crystal molecules having birefringence. Examples of the display mode of an LCD include a twisted nematic (TN) mode in which liquid crystal molecules having a positive dielectric constant anisotropy are spirally aligned along the thickness direction of a liquid crystal layer, a vertical alignment (VA) mode in which liquid crystal molecules having a negative dielectric constant anisotropy are vertically aligned with respect to the substrate surface, an in-plane switching (IPS) mode in which liquid crystal molecules having a positive dielectric constant anisotropy are horizontally aligned with respect to the substrate surface and a horizontal electric field is applied to the liquid crystal layer, and a polymer dispersed liquid crystal (PDLC) mode in which display is performed by dispersing liquid crystal molecules in a polymer matrix to scatter external light.
LCDs, which have advantages in the thin profile, light weight, and low power consumption, are widely used as display devices such as televisions, personal computers, and PDAs. Particularly in recent years, liquid crystal display devices have been rapidly increased in size as seen in liquid crystal display devices for televisions. A multi-domain vertical alignment (MVA) mode is preferably employed in large-sized liquid crystal display devices because large-sized liquid crystal display devices with a wide viewing angle can be produced with high yield. In an MVA mode, liquid crystal molecules having a negative dielectric constant anisotropy are vertically aligned, and a bank (linear protrusion) is provided on a substrate or an opening (slit) of an electrode is provided as an alignment control structure. Alignment control structures in MVA LCDs allow control of the alignment direction of liquid crystals in plural directions under applied voltage even an alignment film is not subjected to rubbing treatment, and the viewing characteristics are better in such a MVA mode than in a conventional TN mode.
However, MVA LCDs disadvantageously provide dark displays. The main cause of this problem is that borders of alignment divisions formed by areas where linear protrusions (ribs) and/or slits generate dark lines. These dark lines reduce the transmittance during the white display and darken the display. Providing greater spacings between the ribs can solve this problem, but this solution results in reduced number of alignment control structures. As a result, it takes longer time to stabilize the alignment after the prescribed voltage is applied to the liquid crystals, which slows down the response speed. A pre-tilt angle imparting technique using a polymer (hereinafter, also referred to as PSA (Polymer Sustained Alignment: alignment sustaining)) has been suggested to improve such a problem and achieve high luminance and high-speed response (see, for example, Patent Literatures 1 to 5).
In the PSA technique, a liquid crystal composition prepared by mixing liquid crystals with polymerizable components such as monomers and oligomers is filled in a space between substrates, the polymerizable components such as the monomers are polymerized into a polymer while a voltage is applied between the substrates to tilt liquid crystal molecules. A layer formed of such a polymer (hereinafter, also referred to as a PSA layer) allows to provide the prescribed pre-tilt angles to the liquid crystals even after elimination of applied voltage, and thus the alignment direction of the liquid crystals can be defined. The polymerizable components such as the monomers are polymerized by heating or light (ultraviolet rays) irradiation. A PSA technique eliminates a need for formation of ribs to improve the aperture ratio, and provides a pre-tilt angle of smaller than 90° to an entire display region to achieve fast response.
Polymer dispersed liquid crystal (PDLC) LCDs, which have a light control layer in which liquid crystals have been dispersed in a polymer matrix, perform display by scattering external light such as natural light or room light. The light control layer is prepared by light irradiation of a material for a light control layer including a liquid crystal composition, a radically polymerizable compound, and a photopolymerization initiator thereby polymerizing the radically polymerizable compound. Formation of a polymer matrix in a light control layer for dispersing liquid crystals in the matrix provides white display under no applied voltage because the refractive indexes of the polymer matrix and the liquid crystals are not agree with each other, and provides black display under applied voltage because the refractive indexes are agree with each other. The polymer matrix is a structure formed to surround liquid crystals, and is inherently different from the PSA layer formed on the substrate as a thin layer (see, for example, Patent Literatures 6 and 7).